Are there any 2 pin 2.54 pitch plastic covers which slide over without any electrical contact only to prevent them from being shorted when not in use?

 

What you use are 2 jumpers, one on each pin with the other half not connected. IE turned 90 degrees from normal use. Or a 2 single or 1 double pin dupont female wire terminal without the wire...

 

I know the connector that @SamR is talking about. If you just need a few, you can use something like https://www.digikey.com/product-detail/en/te-connectivity-amp-connectors/925366-2/A106488-ND/2054446, but you need the pins too. Just insert the pins without wires and crimp them around nothing. I think you can cut the top to the desired length.

I remember these: https://en.wikipedia.org/wiki/Berg_connector

2.5 isn't 2.54, but this https://www.digikey.com/products/en?keywords=A105181-ND should work. Again, you need to buy pins.

 

Nothing to short. It's just a cap.